1. Field of the Invention
This invention relates generally to annunciator circuits and, more particularly, to an annunciator having an improved charging circuit for storing the energy of an applied signal to drive an audible output device.
2. Prior Art Statement
Traditionally, the announcement of an incoming telephone call as been accomplished by a mechanical bell and clapper apparatus energized by the AC portion of a ring signal generated at the telephone switching center. Although such mechanical devices have proven to have exceptional longevity and reliability, it has become increasingly desirable to minimize the number of independant parts comprising a telephone hand set, to realize the resultant savings in assembly and component costs. Accordingly, it has been proposed to utilize solid state ringers, in combination with an appropriate audible output device, to accomplish the annunciation function in a minimum component configuration. Typically, such solid state ringers include an input portion for detecting an incoming ring signal while suppressing undesirable components such as lightening-induced spikes, a charge storage portion for extracting usable energy from the detected ring signal, and an oscillator portion which utilizes the stored energy to drive an audible output device at a frequency and duty cycle determined by the detected ring signal. One such solid state ringer is shown in U.S. Pat. No. 4,163,873, submitted herewith. However, such solid state ringers tend to be less than satisfactory in protecting the sensitive portions thereof from extreme voltage transients, as well as inefficient in harnessing the energy available in the ring signal. Further, the audible output signal provided by such prior art ringers tends to be harsh rather than pleasing due to limitations inherent in the oscillator portions.